Castle on the Coast (UP)

Try to get this before the final showdown versus Aleandra and Vendrick because if you miss it like I did, you'll have to replay the level directly before it to access that part of the castle. To get to the top of the tower, you need to jump through the stained glass floor in the keystone room; platform through that entire area; then go through the portal at the end of it to be spit out in the tower section of the castle that has lava. Jump and climb your way to the tip-top of the tower, above where the final boss fight is on the roof. There will be a big star spire you need to climb to/touch.